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<title>open</title>
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<h2 align=center>open</h2>
<h4 align=center>OS/161 Reference Manual</h4>

<h3>Name</h3>
open - open a file

<h3>Library</h3>
Standard C Library (libc, -lc)

<h3>Synopsis</h3>
#include &lt;unistd.h&gt;<br>
<br>
int<br>
open(const char *<em>filename</em>, int <em>flags</em>);<br>
int<br>
open(const char *<em>filename</em>, int <em>flags</em>, int <em>mode</em>);<br>

<h3>Description</h3>

open opens the file, device, or other kernel object named by the
pathname <em>filename</em>. The <em>flags</em> argument specifies how
to open the file. The optional <em>mode</em> argument is only
meaningful in Unix (or if you choose to implement Unix-style security
later on) and can be ignored.
<p>

The flags argument should consist of one of
<blockquote><table width=90%>
<tr><td>O_RDONLY</td>	<td>Open for reading only.</td></tr>
<tr><td>O_WRONLY</td>	<td>Open for writing only.</td></tr>
<tr><td>O_RDWR</td>		<td>Open for reading and writing.</td></tr>
</table></blockquote>

It may also have any of the following flags OR'd in:
<blockquote><table width=90%>
<tr><td>O_CREAT</td>	<td>Create the file if it doesn't exist.</td></tr>
<tr><td>O_EXCL</td>		<td>Fail if the file already exists.</td></tr>
<tr><td>O_TRUNC</td>	<td>Truncate the file to length 0 upon open.</td></tr>
<tr><td>O_APPEND</td>	<td>Open the file in append mode.</td></tr>
</table></blockquote>

O_EXCL is only meaningful if O_CREAT is also used.
<p>

O_APPEND causes all writes to the file to occur at the end of file, no
matter what gets written to the file by whoever else. (This
functionality may be optional; consult your course's assignments.)
<p>

open returns a file handle suitable for passing to 
<A HREF=read.html>read</A>,
<A HREF=write.html>write</A>,
<A HREF=close.html>close</A>,
etc. This file handle must be greater than or equal to zero.  Note
that file handles 0 (STDIN_FILENO), 1 (STDOUT_FILENO), and 2
(STDERR_FILENO) are used in special ways and are typically assumed by
user-level code to always be open.

<h3>Return Values</h3>
On success, open returns a nonnegative file handle. On error, -1 is
returned, and <A HREF=errno.html>errno</A> is set according to the error
encountered.

<h3>Errors</h3>

The following error codes should be returned under the conditions
given. Other error codes may be returned for other errors not
mentioned here.

<blockquote><table width=90%>
<td width=10%>&nbsp;</td><td>&nbsp;</td></tr>
<tr><td>ENODEV</td>		<td>The device prefix of <em>filename</em> did
				not exist.</td></tr>
<tr><td>ENOTDIR</td>	<td>A non-final component of <em>filename</em>
				was not a directory.</td></tr>
<tr><td>ENOENT</td>		<td>A non-final component of <em>filename</em>
				did not exist.</td></tr>
<tr><td>ENOENT</td>		<td>The named file does not exist, and O_CREAT
				was not specified.</td></tr>
<tr><td>EEXIST</td>		<td>The named file exists, and O_EXCL was
				specified.</td></tr>
<tr><td>EISDIR</td>		<td>The named object is a directory, and it
				was to be opened for writing.</td></tr>
<tr><td>EMFILE</td>		<td>The process's file table was full, or a
				process-specific limit on open files
				was reached.</td></tr>
<tr><td>ENFILE</td>		<td>The system file table is full, if such a
				thing exists, or a system-wide limit
				on open files was reached.</td></tr>
<tr><td>ENXIO</td>		<td>The named object is a block device with no
				mounted filesystem.</td></tr>
<tr><td>ENOSPC</td>		<td>The file was to be created, and the
				filesystem involved is full.</td></tr>
<tr><td>EINVAL</td>		<td><em>flags</em> contained invalid values.</td></tr>
<tr><td>EIO</td>		<td>A hard I/O error occurred.</td></tr>
<tr><td>EFAULT</td>		<td><em>filename</em> was an invalid pointer.</td></tr>
</table></blockquote>

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